Sulfur-based active material

ABSTRACT

The present invention provides a sulfur-based active material prepared using an inexpensive polymer material as a starting material and a method of preparing the sulfur-based active material. A non-aqueous electrolyte secondary battery such as a lithium-ion secondary battery provided with an electrode comprising the sulfur-based active material has a large charging and discharging capacity and an excellent cyclability.

TECHNICAL FIELD

The present invention relates to a sulfur-based active material which is used for an electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery comprising the electrode.

BACKGROUND OF THE INVENTION

WO 2010/044437 discloses a method of manufacturing a lithium-ion secondary battery having a positive electrode formed from a sulfur-based active material obtained by heat-treating a starting material comprising a mixture of polyacrylonitrile and sulfur. Further, JP 6132102 B discloses a method of manufacturing a lithium-ion secondary battery having a positive electrode formed from a sulfur-based active material obtained by heat-treating a starting material comprising a mixture of a diene rubber and sulfur.

SUMMARY OF THE INVENTION

The method described in WO 2010/044437 has a problem that battery performances such as a charging and discharging capacity and cyclability are largely affected by a quality (especially a particle size) of a starting polyacrylonitrile powder, and that a polyacrylonitrile starting powder having a specific quality is very expensive. Meanwhile, the method described in JP 6132102 B uses an inexpensive diene rubber as a starting material, but there a room for improvement from the viewpoint of cyclability.

An object of the present invention is to provide a sulfur-based active material prepared using an inexpensive polymer material as a starting material and a method of preparing the sulfur-based active material. A non-aqueous electrolyte secondary battery such as a lithium-ion secondary battery provided with an electrode comprising the sulfur-based active material has a large charging and discharging capacity and an excellent cyclability.

The present inventors have found that a non-aqueous electrolyte secondary battery which is inexpensive and has a large charging and discharging capacity and excellent cyclability can be manufactured using a novel active material prepared by compounding sulfur and a polymer including methacrylonitrile as a monomer component, and thus, have completed the present invention.

Namely, the present invention relates to:

[1] a sulfur-based active material prepared by baking a starting material comprising sulfur and a polymer including methacrylonitrile as a monomer component,

[2] the sulfur-based active material according to the above [1], wherein the polymer is a copolymer comprising two or more monomer components including methacrylonitrile,

[3] the sulfur-based active material according to the above [1] or [2], wherein the polymer is a copolymer comprising methacrylonitrile, acrylonitrile and other (meth)acrylic monomer as monomer components,

[4] the sulfur-based active material according to any one of the above [1] to [3], wherein the other (meth)acrylic monomer is (meth)acrylic ester,

[5] the sulfur-based active material according to the above [4], characterized in that in a Raman spectrum, there are peaks at around 1,530 cm⁻¹, at around 1,320 cm⁻¹, at around 940 cm⁻¹, at around 470 cm⁻¹, at around 370 cm⁻¹, and at around 310 cm⁻¹,

[6] the sulfur-based active material according to any one of the above [1] to [5], wherein a temperature for baking the starting material is from 250 to 550° C.,

[7] the sulfur-based active material according to any one of the above [1] to [6], wherein the starting material further comprises an electrically conductive additive,

[8] the sulfur-based active material according to the above [7], wherein the electrically conductive additive is an electrically conductive carbon material,

[9] an electrode for a non-aqueous electrolyte secondary battery, the electrode comprising the sulfur-based active material according to any one of the above [1] to [8],

[10] a non-aqueous electrolyte secondary battery comprising the electrode of the above [9], and

[11] a method of preparing a sulfur-based active material, the method comprising a step of baking a starting material comprising sulfur and particles composed of a shell which comprises a polymer including methacrylonitrile as a monomer component and contains hydrocarbon therein.

According to the present invention, it is possible to manufacture a non-aqueous electrolyte secondary battery which is inexpensive, and has a large charging and discharging capacity and excellent cyclability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view schematically illustrating a reaction apparatus used for preparation of a sulfur-based active material.

FIG. 2 is a graph showing a result of a Raman spectrum analysis of the sulfur-based active material obtained in Example 1.

DETAILED DESCRIPTION

A process for manufacturing a non-aqueous electrolyte secondary battery including preparation of a sulfur-based active material according to one embodiment of the present invention is explained below in detail. However, the following description is an example for explaining the present invention, and is not intended to limit a technical range of the present invention within the range of the description. Herein, when a numerical range is shown using “to”, it includes numerical values at both sides thereof.

<Preparation of Sulfur-Based Active Material>

The sulfur-based active material according to the embodiment of the present invention can be prepared by baking a starting material comprising sulfur and a polymer including methacrylonitrile as a monomer component. Further, in a baking step, a vulcanization accelerator and/or an electrically-conductive additive may be further blended to the starting material.

The polymer including methacrylonitrile as a monomer component may be a homopolymer of methacrylonitrile (polymethacrylonitrile) or may be a copolymer of methacrylonitrile with one or more monomer components other than methacrylonitrile.

Known (meth)acrylic monomers such as acrylonitrile, (meth)acrylic acids, (meth)acrylic esters and (meth)acrylamide can be used suitably as the monomer components other than methacrylonitrile. Among these, one or more monomer components selected from the group consisting of acrylonitrile and (meth)acrylic esters are preferable, one or more monomer components selected from the group consisting of acrylonitrile and methyl methacrylate are more preferable, and acrylonitrile and methyl methacrylate are further preferable. Herein, the term “(meth)acryl” stands for “acryl” or “methacryl”.

Further, conjugated diene compounds such as butadiene and isoprene can also be used as the monomer components other than methacrylonitrile.

In the copolymer, a copolymerization ratio of methacrylonitrile is usually from 1 to 99%, preferably from 10 to 95%, more preferably from 20 to 90%, further preferably from 30 to 80%.

In the copolymerization, an order of the copolymerization is not limited particularly, and for example, all monomers may be subjected to random copolymerization at a time, specific monomers may be subjected to copolymerization previously, followed by addition of remaining monomers and copolymerization with each other, or each group of specific monomers may be subjected to copolymerization previously, followed by block copolymerization.

The polymerization can be performed by usual polymerization methods such as an anion polymerization reaction, coordination polymerization reaction and the like. The polymerization method is not limited particularly, and any of a solution polymerization method, an emulsion polymerization method, a gas phase polymerization method and a bulk polymerization method can be used. Further, the polymerization may be any of batch processing and continuous processing.

A weight-average molecular weight (Mw) of the polymer is preferably from 1,000 to 1,000,000, more preferably from 10,000 to 300,000. It is noted that the Mw can be calibrated with standard polystyrene based on measurement values determined with a gel permeation chromatography (GPC) (GPC-8000 series manufactured by Tosoh Corporation; detector: differential refractometer; column: TSKGEL SUPERMALTPORE HZ-M manufactured by Tosoh Corporation).

Powdery sulfur, insoluble sulfur and precipitated sulfur can be used as sulfur, but colloidal sulfur is preferable since sulfur is in a form of fine particles. The compounding amount of sulfur is preferably not less than 250 parts by mass, more preferably not less than 300 parts by mass based on 100 parts by mass of the polymer from the viewpoint of a charging and discharging capacity and cyclability. While there is no upper limit of the compounding amount of sulfur, the compounding amount is preferably not more than 2,000 parts by mass, more preferably not more than 1,500 parts by mass since if the amount exceeds 1,500 parts by mass, a charging and discharging capacity or cyclability is saturated, and the amount of not more than 1,500 parts by mass is advantageous from the viewpoint of cost.

When compounding a vulcanization accelerator, its amount is preferably 3 parts by mass or more, more preferably 10 parts by mass or more based on 100 parts by mass of the thermally expanding micro capsules from the viewpoint of a charging and discharging capacity and cyclability. While there is no upper limit of the amount of the vulcanization accelerator, the amount is preferably not more than 250 parts by mass, more preferably not more than 50 parts by mass since if the ratio exceeds 250 parts by mass, a charging and discharging capacity or cyclability is saturated, and the amount of not more than 250 parts by mass is advantageous from the viewpoint of cost.

The electrically-conductive additive is not limited particularly, and electrically-conductive carbon materials such as vapor grown carbon fibers (Vapor Grown Carbon Fibers: VGCF), carbon powders, carbon black (CB), acetylene black (AB), KETJENBLACK (KB) and graphite can be used suitably. From the viewpoint of a capacity density, an input/output characteristic and electric conductivity, acetylene black (AB) or KETJENBLACK (KB) is preferable. These electrically-conductive additives may be used alone or may be used in combination of two or more thereof.

When compounding the electrically-conductive additive, its amount is preferably 1 part by mass or more, more preferably 3 parts by mass or more, further preferably 5 parts by mass or more based on 100 parts by mass of the polymer from the viewpoint of a charging and discharging capacity and cyclability. Meanwhile, the amount is preferably 50 parts by mass or less, more preferably 30 parts by mass or less, further preferably 20 parts by mass or less. When the amount is more than 50 parts by mass, a ratio of a sulfur-containing structure in the sulfur-containing compound decreases relatively. Therefore, there is a tendency that an object of further improving a charging and discharging capacity and cyclability is hardly achieved.

Baking of the starting material comprising the above-mentioned components is performed while heating the starting material under a non-oxidizing atmosphere. The non-oxidizing atmosphere is an atmosphere substantially including no oxygen and is used to inhibit deterioration of ingredient components due to oxidation and excessive thermal decomposition. Specifically the starting material is subjected to heating in a quartz tube filled with an inert gas such as a nitrogen gas or an argon gas under an inert gas atmosphere. A baking temperature is preferably 250° C. or higher, more preferably 300° C. or higher. When the baking temperature is lower than 250° C., there is a tendency that a sulfurization reaction is insufficient and a charging and discharging capacity of a target product becomes lower. Meanwhile, the baking temperature is preferably 550° C. or lower, more preferably 500° C. or lower, further preferably 450° C. or lower. When the baking temperature exceeds 550° C., decomposition of the starting compound increases, resulting in a decreased yield and a reduced charging and discharging capacity.

So-called unreacted sulfur such as sulfur resulting from cooling and precipitation of sulfur sublimated during the baking remains in sulfides obtained after the baking. Since sulfur is an insulator, it functions as an electric resistance in the electrode and may cause decrease of a battery performance. Since the unreacted sulfur becomes a factor for decreasing cyclability, it is necessary to remove the unreacted sulfur. The unreacted sulfur can be removed by a method such as a method of drying by heating under reduced pressure, a method of drying with hot air or a method of washing with a solvent.

The obtained sulfur-based active material can be pulverized and classified into a predetermined particle size to obtain particles having a particle size suitable for manufacturing an electrode. A preferred particle size distribution of the particles is from about 5 to 25 μm in a median size. It should be noted that in a baking method using a biaxial extruder, pulverization of the prepared sulfur-based active material also can be carried out by shearing during the kneading at the same time as the preparation of the sulfur-based active material.

As a total content of sulfur in the sulfur-based active material increases, cyclability of a non-aqueous electrolyte secondary battery tends to be enhanced, and therefore, the total content of sulfur in the sulfur-based active material, which is measured by an elemental analysis, is preferably 35% by mass or more, more preferably 40% by mass or more. Further, the total content of oxygen in the sulfur-based active material, which is measured by an elemental analysis, is preferably 3% by mass or more, more preferably 4% by mass or more.

The sulfur-based active material according to the embodiment of the present invention is characterized in that there are peaks at around 1,530 cm⁻¹, at around 1,320 cm⁻¹, at around 940 cm⁻¹, at around 470 cm⁻¹, at around 370 cm⁻¹, and at around 310 cm⁻¹ within a range of from 200 cm⁻¹ to 1,800 cm⁻¹, in a Raman spectrum. These peaks in a Raman shift are observed in the same peaks as in the case where a ratio of sulfur atoms to the polymer is changed, and characterize the sulfur-based active material according to the embodiment of the present invention. Each of the peaks can exist substantially within an error range of ±8 cm⁻¹ with respect to each of the peaks. It is noted that the above-mentioned Raman shift is one measured using a RAMANtouch manufactured by Nanophoton Corporation (an excitation wavelength λ=532 nm, a grating: 1200 gr/mm, a resolution: 1.2 cm⁻¹).

When a sulfur and a polymer including methacrylonitrile as a monomer component are mixed and heated at the predetermined temperature, a ring-closing reaction occurs and sulfur is taken into the polymer to form a three-dimensionally crosslinked structure. Thus, the sulfur-based active material according to the embodiment of the present invention obtained in such way suppresses the elution to electrolyte solution during a charging and discharging cycle. Thus, cyclability of the non-aqueous electrolyte secondary battery using the sulfur-based active material on an electrode is enhanced.

<Configuration of Electrode>

The electrodes (positive electrode and negative electrode) for the non-aqueous electrolyte secondary battery according to the embodiment of the present invention can have the same structure as in general non-aqueous electrolyte storage batteries. For example, the electrodes for the non-aqueous electrolyte secondary battery according to the embodiment of the present invention can be produced by applying, to a current collector, an electrode slurry prepared by mixing the above-mentioned sulfur-based active material, a binder, an electrically conductive additive and a solvent. Further, the electrodes can be produced by another method of kneading a mixture of the sulfur-based active material, the electrically conductive additive and the binder with a mortar, a press or the like, forming the mixture into a film and applying the mixture in the form of film onto a current collector with a press or the like.

(Current Collector)

As for a current collector, those which have been used commonly on electrodes for lithium-ion secondary batteries may be used. Examples of the current collector include aluminum-based current collectors such as aluminum foils, aluminum meshes, punched aluminum sheets and aluminum expanded sheets; stainless-steel-based current collectors such as stainless-steel foils, stainless-steel meshes, punched stainless-steel sheets and stainless-steel expanded sheets; nickel-based current collectors such as foamed nickel and nickel nonwoven fabrics; copper-based current collectors such as copper foils, copper meshes, punched copper sheets and copper expanded sheets; titanium-based current collectors such as titanium foils and titanium meshes; and carbon-based current collectors such as carbon nonwoven fabrics and carbon woven fabrics. Among these, aluminum-based current collectors are preferable from the viewpoint of a mechanical strength, conductivity, a mass density, cost and the like.

There is particularly no limitation in a shape of the current collector, and for example, beaten substrates, three-dimensional substrates and the like can be used. When three-dimensional substrates (foamed metal, mesh, woven fabric, non-woven fabric, expanded substrate and the like) are used, even in the case of a binder lacking adhesion to the current collector, there is a tendency that an electrode having a high capacity density can be obtained and a high rate charging and discharging characteristic becomes satisfactory.

(Binder)

Known binders which are used on electrodes can be used as a binder, and from the viewpoint of affinity for water and reduction of environmental load, aqueous binders are suitably used. Examples of the aqueous binder include hydroxypropyl cellulose (HPC), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), acrylic resins, styrene-butadiene rubber (SBR), water-soluble polyimide (PI), water-soluble polyamide-imide (PAI), methacrylic resins (PMA), polyethylene oxide (PEO), urethane and the like. One or more thereof can be used alone or can be used in combination.

(Electrically-Conductive Additive)

The above-mentioned electrically-conductive additives usable in manufacturing the sulfur-based active material can be used similarly as the electrically-conductive additive.

(Solvent)

A solvent (aqueous solvent) containing water is preferable and water is preferable as a solvent for dispersing solid components such as the sulfur-based active material, the binder and the electrically conductive additive in producing the electrode slurry. If an organic solvent other than water is used, there is a tendency that elution, from the sulfur-based active material, of a sulfur component contributing to a charging and discharging reaction occurs and a charging and discharging capacity of the battery decreases. Further, an aqueous solvent is preferable also from the viewpoint of reduction of environmental load. In addition, within a range of not impairing the effect of the present invention (for example, an amount of an organic solvent other than water is less than 20% by mass), a solvent mixing with water such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylformaldehyde or lower alcohols may be mixed.

A content of the sulfur-based active material is preferably 85% by mass or more, more preferably 87% by mass or more, further preferably 90% by mass or more based on 100% by mas of the solid components (particularly the sulfur-based active material, the binder and the electrically conductive additive. hereinafter the same) in the electrode slurry. Further, an upper limit of the content of the sulfur-based active material is not restricted, and is preferably 99% by mass or less, more preferably 97% by mass or less, further preferably 95% by mass or less.

A content of the binder is preferably from 0.1 to 10.0% by mass, more preferably from 0.5 to 8.0% by mass, further preferably from 1.0 to 6.0% by mass, particularly preferably from 2.0 to 5.0% by mass based on 100% by mass of the solid components in the electrode slurry.

When compounding the electrically conductive additive to the electrode slurry, a content thereof is preferably from 0.1 to 10.0% by mass, more preferably from 0.5 to 8.0% by mass, further preferably from 1.0 to 6.0% by mass, particularly preferably from 2.0 to 5.0% by mass based on 100% by mas of the solid components in the electrode slurry.

(Electrode Material)

When using the sulfur-based active material according to the embodiment of the present invention on a positive electrode, examples of a negative electrode material include known metallic lithium, carbon-based materials such as graphite, silicon-based materials such as a silicon thin film and SiO, tin alloy-based materials such as copper-tin and cobalt-tin and the like. In the case where a carbon-based material, a silicon-based material, a tin alloy-based material or the like that does not include lithium is used, short-circuiting between positive and negative electrodes, which results from production of dendrite, can be less likely to arise, and a long service life of the non-aqueous electrolyte secondary battery can be achieved. Among the above-mentioned negative electrode materials, silicon-based materials, which are high capacity negative electrode materials, are preferred, and a silicon thin film that can make a thickness of the electrode thinner and is advantageous in capacity per volume is more preferable.

However, in the case where a negative electrode material that does not include lithium is used in combination with the positive electrode according to the embodiment of the present invention, neither the positive electrode nor the negative electrode includes lithium and thus a pre-doping treatment, in which lithium is inserted into either one of the negative electrode or positive electrode, or into both of them, becomes necessary.

For a method of lithium pre-doping, a publicly known method can be used. For example, in the case where a negative electrode is doped with lithium, the following methods of inserting lithium can be given: an electrolytically-doping method, in which a half-cell is assembled using metallic lithium as the counter electrode and then doping lithium electrochemically; and an application pre-doping method, in which doping is done by a diffusion of lithium onto an electrode by applying a metallic lithium foil onto the electrode and then leaving the electrode with the metallic lithium foil applied as it is within an electrolytic solution. Moreover, in another case as well where the positive electrode is pre-doped with lithium, it is possible to utilize the aforementioned electrolytically-doping method.

When using the sulfur-based active material according to the embodiment of the present invention on a negative electrode, examples of a positive electrode material include lithium-transition metal composite oxides (particularly cobalt-based composite oxides, nickel-based composite oxides, manganese-based composite oxides, cobalt-nickel-manganese ternary composite oxides). Further, lithium transition metal phosphate having an olivine type crystal structure (particularly lithium iron phosphate, lithium manganese phosphate) and the like can also be used. In addition, in the case where a positive electrode produced using a lithium-containing transition metal-lithium composite oxide-based compound as an active material is used in combination with a negative electrode produced using the electrode slurry according to the embodiment of the present invention, a lithium pre-doping treatment is not always necessary since the positive electrode includes lithium.

<Electrolyte>

An electrolyte constituting a non-aqueous electrolyte secondary battery may be a liquid or solid having ionic conductivity, and the same electrolyte as one used on known non-aqueous electrolyte secondary batteries can be used. However, from the viewpoint of high output characteristics, it is possible to use those in which an alkali-metal salt serving as a supporting electrolyte is dissolved in an organic solvent.

Examples of an organic solvent include at least one selected from non-aqueous solvents such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethyl ether, γ-butyrolactone, and acetonitrile. Preferred as an organic solvent is ethylene carbonate, propylene carbonate or a solvent mixture thereof.

Examples of the supporting electrolyte include LiPF₆, LiBF₄, LiAsF₆, LiCF₃SO₃, LiI, LiClO₄ and the like, and LiPF₆ is preferred.

A concentration of the supporting electrolyte can be from about 0.5 mol/liter to 1.7 mol/liter. It is noted that the electrolyte is not limited to a liquid form. For example, in the case where the non-aqueous electrolyte secondary battery is a lithium polymer secondary battery, the electrolyte can be a solid form (for example, a form of polymer gel), an ionic liquid, a molten salt or the like.

In addition to the above-described positive electrode, negative electrode and electrolyte, the non-aqueous electrolyte secondary battery can be further equipped with the other members, such as separators, as well. A separator intervenes between the positive electrode and the negative electrode, thereby not only allowing the movements of ions between the both electrodes but also preventing the positive electrode and the negative electrode from internally short-circuiting one another. When the non-aqueous electrolyte secondary battery is a hermetically-closed type, a function of retaining the electrolytic solution is required for the separator.

As for a separator, it is preferable to use a thin-thickness and microporous or nonwoven-shaped film that is made of a material such as polyethylene, polypropylene, polyacrylonitrile, aramid, polyimide, cellulose, glass or the like.

A configuration of the non-aqueous electrolyte secondary battery according to the embodiment of the present invention is not limited particularly, and can be formed as a variety of configurations such as cylindrical types, laminated types, coin types, button types and the like.

The non-aqueous electrolyte secondary battery provided with the electrode according to the embodiment of the present invention has a high capacity and is excellent in cyclability, and therefore, can be used as power sources for electrical devices such as a smartphone, power tools, vehicles, UPS and the like.

EXAMPLE

The present invention is explained by means of Examples, but is not limited to the Examples.

Various chemicals used in Examples and Comparative Examples are shown below.

Polymer 1: Methacrylonitrile-acrylonitrile-methyl methacrylate copolymer

Polymer 2: Acrylonitrile-methyl methacrylate copolymer

Polymer 3: Acrylonitrile-butadiene copolymer

Sulfur: Precipitated sulfur available from Tsurumi Chemical Industry Co., Ltd.

Electrically conductive additive: Acetylene black (Denka Black available from Denka Company Limited)

Example 1

(Preparation of Starting Material)

A starting material is obtained by compounding Polymer 1 and sulfur according to a compounding formulation of Example 1 shown in Table 1 and cutting a mixture into small pieces using a cutter mill, followed by a baking step.

(Reaction Apparatus)

A reaction apparatus 1 as illustrated in FIG. 1 was used for baking of the starting compound. The reaction apparatus 1 comprises a reaction container 3, which has an outer diameter of 60 mm, an inner diameter of 50 mm and a height of 300 mm and is made of quartz glass, that is formed as a bottomed cylindrical shape to contain and baking the starting compound 2; a silicone plug 4 for closing an upper opening of the reaction container 3; one alumina protection tube 5 (“Alumina SSA-S” available from NIKKATO CORPORATION, an outer diameter of 4 mm, an inner diameter of 2 mm and a length of 250 mm) and two tubes, which are a gas introducing tube 6 and a gas exhausting tube 7 (both are “Alumina SSA-S” available from NIKKATO CORPORATION, an outer diameter of 6 mm, an inner diameter of 4 mm and a length of 150 mm), these three tubes penetrating through the plug 4; and an electric furnace 8 (crucible furnace, width of an opening: 80 mm dia., heating height: 100 mm) for heating the reaction container 3 from the bottom side.

The alumina protection tube 5 is formed in such a length that the lower part below plug 4 reaches the starting compound 2 contained in the bottom of the reaction container 3 and a thermocouple 9 is inserted through the inside of the alumina protection tube 5. The alumina protection tube 5 is used as a protective tube for the thermocouple 9. The leading end of the thermocouple 9 is inserted into the starting compound 2 while being protected by the closed leading end of the alumina protection tube 5 and functions to measure a temperature of the starting compound 2. Output of the thermocouple 9 is input in a temperature controller 10 of the electric furnace 8 as shown by the solid arrow in the drawing and the temperature controller 10 functions to control a heating temperature of the electric furnace 8 based on the input from the thermocouple 9.

The gas introducing tube 6 and the gas exhausting tube 7 are formed such that the bottom end thereof projects in 3 mm downwardly from the plug 4.

The gas introducing tube 6 is continuously supplied with Ar gas from a gas supply system which is not shown. The gas exhausting tube 7 is connected to a trapping bath 12 containing an aqueous solution 11 of sodium hydroxide. The exhaust gas moving toward the outside through the gas exhausting tube 7 from the reaction container 3 is released to the outside after passing through the aqueous solution 11 of sodium hydroxide in the trapping bath 12. Therefore, even if hydrogen sulfide gas generated from a vulcanization reaction is included in the exhaust gas, the hydrogen sulfide gas is removed therefrom by being neutralized with the aqueous solution of sodium hydroxide.

(Baking Step)

In the baking step, heating with the electric furnace 8 was started 30 minutes after starting a continuous supply of Ar gas to the reaction container 3 holding the starting compound 2 in its bottom at a flow rate of 80 ml/min from the gas supply system. The temperature elevation rate was 150° C./min. When the temperature of the starting compound reached 450° C., baking was conducted for two hours while maintaining the temperature of 450° C. Then, the starting compound 2 was cooled naturally under an Ar gas atmosphere to 25° C. while adjusting the flow rate of the Ar gas and a reaction product was taken out of the reaction container 3.

(Removal of Unreacted Sulfur)

In order to remove the unreacted sulfur (free elemental sulfur) remaining in the product after the baking step, the following step was carried out. Namely, the product was pulverized in a mortar and 2 g of a pulverized product was put in a glass tube oven and heated for three hours at 250° C. while vacuum suction was conducted to produce a sulfur-based active material in which unreacted sulfur was removed (or only a trace amount of unreacted sulfur was contained). The temperature elevation rate was 10° C./min.

(Raman Spectrum Analysis)

With respect to an obtained sulfur-based active material, a Raman spectrum analysis was carried out using a RAMANtouch manufactured by Nanophoton Corporation under a condition of an excitation wavelength λ=532 nm, a grating: 1200 gr/mm, and a resolution: 1.2 cm⁻¹ (FIG. 2). FIG. 2 shows a relative strength in a vertical axis and a Raman shift (cm⁻¹) in a horizontal axis. In the obtained sulfur-based active material, peaks are observed at around 1,530 cm⁻¹, at around 1,320 cm⁻¹, at around 940 cm⁻¹, at around 470 cm⁻¹, at around 370 cm⁻¹, and at around 310 cm⁻¹ within a range of from 200 cm⁻¹ to 800 cm⁻¹ in a Raman shift.

<Elemental Analysis>

As for carbon, hydrogen and nitrogen, a mass ratio (%) based on a total amount of a sulfur-based active material was calculated from a mass amount measured with a full automatic elemental analysis device, vario MICRO cube manufactured by Elementar Analysensysteme GmbH. As for sulfur, a mass ratio (%) based on a total amount of a sulfur-based active material was calculated from a mass amount measured with an ion chromatograph device DX-320 manufactured by Dionex Corporation using a column (IonPac AS12A) manufactured by the same Corporation. For oxygen, a mass ratio (%) was calculated by an inert gas impulse heat dissolution NDIR method using EMGA-920 manufactured by Horiba, Ltd.

<Preparation of Lithium-Ion Secondary Battery>

[1] Positive Electrode

The sulfur-based active material as produced above, an electrically conductive additive and an aqueous acrylic resin were measured so that the compounding ratio thereof became 90:5:5 (mass ratio), and were put into a container. By using Milli-Q ultra pure water as a dispersant, a mixture was subjected to stirring and mixing with a rotation/revolution mixer (ARE-250 available from Thinky Corporation) to prepare a uniform slurry. The prepared slurry was applied onto a 20 μm thick aluminum foil with an applicator having a slit width of 60 μm, followed by compression with a roll press and then 3-hour heating at 120° C. with a dryer. After the drying, an obtained foil was punched into a disc of 11 mm diameter to obtain a positive electrode for a lithium-ion secondary battery. Thereafter, a weight of the positive electrode was measured, and a mass ratio of the active material in the electrode was calculated from the above-mentioned ratio.

[2] Negative Electrode

A metallic lithium foil (manufactured by Honjo Metal Co., Ltd.) having a thickness of 500 μm and punched out to a circle with 14 mm in diameter was used as a negative electrode.

[3] Electrolyte

A non-aqueous electrolyte in which LiPF₆ had been dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate was used as an electrolyte. A volume ratio of ethylene carbonate and diethyl carbonate was 1:1. A concentration of LiPF₆ in the electrolyte was 1.0 mol/liter.

[4] Battery

Using the positive electrode and negative electrode obtained in the above [1] and [2], a coin-type battery was prepared. Specifically, in a dry room, a separator (Celgard 2400 manufactured by Celgard, a polypropylene microporous film with a thickness of 25 μm) and a glass nonwoven filter (GA100 manufactured by ADVANTEC CO., LTD. and having a thickness of 440 μm) were sandwiched between the positive electrode and the negative electrode to form an electrode-assembly battery. This electrode-assembly battery was accommodated in a battery case (a member for CR2032-type coin battery, a product of HOSEN Co., Ltd.) made of a stainless-steel container and the electrolyte solution prepared in the above [3] was added to the battery case. The battery case was sealed hermetically with a crimping machine, thereby obtaining a coin-type lithium-ion secondary battery of Example 1.

Example 2

A battery was produced in the same manner as in Example 1 except that an electrically conductive additive was further blended to a starting material.

Comparative Examples 1 and 2

A battery was produced in the same manner as in Example 1 except that Polymer 2 or 3 was used instead of Polymer 1.

[Test Method]

<Measuring Test of Charging and Discharging Capacity>

With respect to each coin-type lithium-ion secondary battery prepared in Examples and Comparative Examples, charging and discharging were carried out at an electric-current value equivalent to 50 mA per 1 g of the sulfur-based active material under a condition of a test temperature of 30° C. The discharge termination voltage was 1.0 V and the charging termination voltage was 3.0 V. Charging and discharging was repeated 30 times. Each discharging capacity (mAh/g) was measured and a discharging capacity (mAh/g) at the second discharging was regarded as an initial capacity. The results are shown in Table 1. The larger the initial capacity is, the larger the charging and discharging capacity of the lithium-ion secondary battery is, which is evaluated as preferable.

Moreover, from a discharging capacity DC₁₀ (mAh/g) at the tenth discharging and a discharging capacity DC₃₀ (mAh/g) at the thirtieth discharging, a capacity retention rate (%) was calculated by the following formula. The results are shown in Table 1. It can be said that the higher the capacity retention rate is, the more excellent cyclability of the lithium-ion secondary battery is. Capacity retention rate (%)=(DC₃₀(mAh/g)/DC₁₀(mAh/g))×100

TABLE 1 Example Comparative Example 1 2 1 2 Positive electrode active material Formulation (part by mass) Polymer 1 100 100 — — Polymer 2 — — 100 — Polymer 3 — — — 100 Sulfur 600 600 600 500 Electrically conductive additive — 12 — — Elemental analysis (%) C 35.6 40.1 48.2 41.6 H 0.8 0.6 1.9 0.2 N 12.4 11.3 12.6 4.2 S 47.5 43.1 26.2 53.8 O 5.4 5.0 12.0 0.2 Evaluation of battery Discharging capacity (mAh/g) First time 849 843 427 522 Second time 639 653 242 267 Tenth time 616 619 233 241 Thirtieth time 585 589 227 216 Capacity retention rate (%) 95.0 95.2 97.4 89.6

From the results of Table 1, it is seen that the lithium-ion secondary battery produced using, as an electrode material, the sulfur-based active material of the present invention prepared by compounding sulfur and a polymer including methacrylonitrile as a monomer component.

INDUSTRIAL APPLICABILITY

A non-aqueous electrolyte secondary battery which is inexpensive, assures easy manufacturing, and has a large charging and discharging capacity and excellent cyclability can be manufactured by using a sulfur-based active material of the present invention as an electrode material.

EXPLANATION OF SYMBOLS

-   1 Reaction apparatus -   2 Starting compound -   3 Reaction container -   4 Silicone plug -   5 Alumina protection tube -   6 Gas introducing tube -   7 Gas exhausting tube -   8 Electric furnace -   9 Thermocouple -   10 Temperature controller -   11 Aqueous solution of sodium hydroxide -   12 Trapping bath 

What is claimed is:
 1. A sulfur-based active material prepared by baking a starting material comprising: elemental sulfur and a copolymer comprising methacrylonitrile, acrylonitrile and (meth)acrylic ester as monomer components.
 2. The sulfur-based active material of claim 1, characterized in that in a Raman spectrum, there are peaks at around 1,530 cm⁻¹, at around 1,320 cm⁻¹, at around 940 cm⁻¹, at around 470 cm⁻¹, at around 370 cm⁻¹, and at around 310 cm⁻¹.
 3. The sulfur-based active material of claim 1, wherein a temperature for baking the starting material is from 250 to 550° C.
 4. The sulfur-based active material of claim 1, wherein the starting material further comprises an electrically conductive additive.
 5. The sulfur-based active material of claim 4, wherein the electrically conductive additive is an electrically conductive carbon material.
 6. The sulfur-based active material of claim 1, wherein the total content of sulfur in the sulfur-based active material is 35% by mass or more as measured by elemental analysis.
 7. The sulfur-based active material of claim 1, wherein the sulfur-based active material is composed of five elements, C, H, N, S and O.
 8. The sulfur-based active material of claim 1, wherein the copolymer is a methacrylonitrile-acrylonitrile-methyl methacrylate copolymer.
 9. The sulfur-based active material of claim 1, wherein the weight ratio of the copolymer to the elemental sulfur is in the range of 100 parts by mass of the copolymer to 250 to 2,000 parts by mass of the elemental sulfur.
 10. The sulfur-based active material of claim 1, wherein the weight ratio of the copolymer to the elemental sulfur is in the range of 100 parts by mass of the copolymer to 300 to 1,500 parts by mass of the elemental sulfur.
 11. An electrode for a non-aqueous electrolyte secondary battery, the electrode comprising the sulfur-based active material of claim
 1. 12. A non-aqueous electrolyte secondary battery comprising the electrode of claim
 11. 13. A method of preparing a sulfur-based active material, the method comprising a step of baking a starting material comprising elemental sulfur and a polymer including methacrylonitrile as a monomer component. 